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warping in stainless steel

Any advice would be appreciated
Two years ago I bought a steel plate, 12C27. It was about 1m X 38 cm X 3,75 mm. It was quite flat when I bought it, perhaps with the smallest faint of a radius lenghtwise but nothing that was noticable in the blades I cut out. But with time it becomes more and more warped, both lengthways and sideways. The other day when I was cutting it, warpage was to the degree that I had to straighten out the blade which was cut across the short side.

How can it be that a SS plate suddenly warps? My candidates are:
1.I have been storing it lying on the shed floor and that temperature differences in combination with tensions in the material affect it.
2. When I´m cutting it with my angle grinder I reach temperatures that cause this. Edges of the cuts appear to be hardened about half a milimetre or so.(the darn machine itself caught fire last time so it wont do that again)
3. I have been fooling myself all the time and the steel is no different than it was when I bought it.

A)And what should I do with it? B)Shall I try to anneal it by tossing it in the barbecue so as not to warp in again when is is being sent to HT? Or will simply straighten the blades out suffice? C)When I buy another angle grinder are there some techniques for cutting with it that are better than other for the steel, (or indeed the machine itself )?

Re: warping in stainless steel

Localised hardening Torsten, if you look at the affect on a katana of edge hardening, applying this to two or more sides will likely affect it the way you experienced, especially if there was already a biase

Re: warping in stainless steel

Thank you, kindly!
At least I´m not imagining it all then. Should I simply accept this as a part of the making of a knife or should I go back to using a hacksaw? Should I cool the steel while cutting it? would it make a difference if I heated it all once?

Re: warping in stainless steel

steel warps all the time, dosn't really matter where it comes from and how its stored. Don't stick it in a barbie unless you want knifingmaking to be more unpredictable than its already is. Just cut out the blade and straighten it before grinding and you might have to straigten after grinding as well and before its heat treated. Its worth asking your heat treater if they will straighten the blade after hardening but before tempering.

Re: warping in stainless steel

I thank you both for your valid answers. I feel much more secure after reading them.
I have actually gotten a few blades back from HT with a small curvation and dont think they will do much manual handling of the blades at all. I believe that whoever I send my blades to in this country, they will in turn send them to the same place for the job.